LA CROSSE, Wis. (AP)  It wasn't rain and definitely not snow that showed up as a cloud on the radar last week. The National Weather Service said Thursday it was a gigantic hatch of mayflies.

The mayflies started hatching around 9 p.m. June 30 and continued overnight along the Mississippi River valley, according to the Weather Service.

"Some roads across the Mississippi River in and around La Crosse were covered with bugs, piling into 'drifts' on bridges over the Mississippi River and its tributaries," the National Weather Service reported. "Local businesses with high intensity lighting soon found large piles of dead mayflies accumulating under the lights by midnight."

Lake Huron and other areas have experienced large numbers of mayflies in recent years, said Mike Miller, a stream water ecologist with the Wisconsin Department of Natural Resources.

The big hatches might mean water quality has improved in the Mississippi and other waters, he said.

The bugs burrow in lake and river bottoms until they mature. They surface, mate, lay eggs and die.

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This radar image released by the National Weather Service shows a gigantic hatch of mayflies along the Mississippi River in counties from Wisconsin, Iowa and Minnesota. The mayflies started hatching around 9 p.m. June 30 and continued overnight along the Mississippi River valley.